The power limits of a supercharging system such as an exhaust gas turbocharger are increased, for example by regulated two-stage supercharging, as is known from Bosch, Kraftfahrtechnisches Taschenbuch [Automotive Handbook], 23rd Edition, Vieweg, 1999, pages 445 through 446. In regulated two-stage supercharging, two exhaust gas turbochargers of different sizes are connected in series. The exhaust gas mass flow first flows into an exhaust gas manifold. From there, the exhaust gas mass flow is expanded via a high pressure turbine. In the case of large exhaust gas volumes, which can occur at high speeds, a portion of the exhaust gas mass flow may be redirected around the high pressure turbine via a bypass. The entire exhaust gas mass flow is subsequently used by a low pressure turbine connected downstream from the high pressure turbine. The incoming fresh air mass flow is first precompressed by a low pressure stage and then further compressed in the high pressure stage. Ideally, the fresh air mass flow undergoes intermediate cooling between the low pressure stage and the high pressure stage.
At approximately 50% to 60% of the nominal speed, the exhaust gas is completely redirected around the high pressure turbine via a bypass. As a result, the high pressure compressor, which is driven by the high pressure turbine and is connected in series to a low pressure compressor driven by the low pressure turbine, is simultaneously taken out of operation. In this case, the high pressure compressor is bypassed via a charge air line in which a nonreturn valve is provided to prevent charge air from flowing back via the charge air line during operation of the high pressure compressor.
Two-stage supercharging in a supercharging system is generally carried out by two series-connected exhaust gas turbochargers. This achieves a two-stage expansion via the two turbine parts of the two exhaust gas turbochargers as well as a two-stage compression on the compressor side of the two series-connected exhaust gas turbochargers. The disadvantages of unregulated two-stage supercharging are avoided by regulating devices for bypassing the high pressure turbine and high pressure compressor.
Cutting off the exhaust gas mass flow upstream from the high pressure turbine regulates the power of the high pressure turbine. The exhaust gas mass flow leaving the high pressure turbine mixes with a portion of the exhaust gas mass flow flowing through the bypass valve and is subsequently expanded in the low pressure turbine. A disadvantage of this procedure is the fact that the difference in pressure present between the outlet side of the internal combustion engine and the output of the high pressure turbine is expanded by a bypass valve without reducing work. In the case of unregulated two-stage supercharging, the fact that the entire exhaust gas mass flow is expanded in an unregulated manner in the high pressure and low pressure turbines is disadvantageous. This means that the power of the two-stage supercharging system rises in an unregulatable manner between a specific load point and the maximum load point, which is unsuitable for use in an internal combustion engine of a passenger car. Due to the design of the high pressure and low pressure turbines in the case of unregulated two-stage supercharging, an unsatisfactory response characteristic exists in the operating range up to and above the design point.
European Patent Application No. EP 0 718 481 relates to an exhaust gas recirculation system for a supercharged internal combustion engine. To lower exhaust emissions, in particular to substantially reduce NOx levels in the partial load range of the internal combustion engine, a method is known for recirculating a portion of the exhaust gas to suppress NOx formation by reducing the oxygen content. In supercharged engines, recirculating the exhaust gas directly results in a noticeable decrease in the power of the exhaust gas turbine. This decrease is even more pronounced in a two-stage supercharging system. To avoid power decreases of this type, which usually go hand-in-hand with increased fuel consumption, a method is described in which the exhaust gas to be recirculated is removed from between the high pressure turbine part and the low pressure turbine part and supplied to the inlet of the low pressure compressor which is higher in relation to the low pressure turbine. This ensures an exhaust gas recirculation method which minimizes increased fuel consumption.
The design of a two-stage supercharging system poses a particular difficulty in internal combustion engines having a V-shaped cylinder layout, because the two cylinder banks must be uniformly impinged upon by the turbochargers. If only one turbocharger is used as the high pressure stage and one turbocharger as the low pressure stage, the supercharging system may be positioned in front of or behind the internal combustion engine without taking up a lot of space. Although this provides a number of advantages, it nevertheless has the disadvantage that there is not always enough room in the engine compartment.